Near central station, the Leidseplein, and the Rembrandtsplein.
If for some reason you don't want falafel (maybe you have a date later and don't want the garlic?), you can get a sandwich (broodje), soup, or something light at any cafe in town. The French fries are great in most places too (that is "patat" or "friet"). Unless you are British in which case you want soggy "chips" and then you are pretty much screwed; wait until you get back to the UK! ;)